1. Field of the Invention
The present invention relates to a toner for electrophotography to be used in developing electrical or magnetic latent images in electrophotography, electrostatic printing, and the like.
2. Description of the Related Art
Toners for developing electrical or magnetic latent images, and the like are used in various processes for forming and recording images.
One of such image forming processes is electrophotography, a variety of which are available, as shown, for example, in U.S. Pat. No. 2,297,691.
In electrophotography, which uses a photosensitive member generally formed of a photoconductive material, an electrical latent image is formed on the photosensitive member by various means. The electrical latent image is developed by using a toner. The toner image thus obtained is transferred to a recording material, such as paper, and then fixed thereto by heating or pressurization or by using solvent vapor or the like, thereby obtaining a copy of the image. Where a process for transferring toner images to recording material is included, there is usually also provided a process for removing the toner remaining on the photosensitive member.
The following are examples of developers conventionally used in dry development devices for electrophotography:
1: One-component-type magnetic developer comprising a toner containing magnetic powder. PA1 2: One-component-type non-magnetic developer comprising a toner containing no magnetic powder. PA1 3: Two-component-type non-magnetic developer comprising a toner containing no magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion. PA1 4: Two-component-type magnetic developer comprising a toner containing magnetic powder and a magnetic carrier, which is mixed with the toner in a fixed proportion. PA1 a first toner A comprising a toner particle group (a) having a first particle size distribution and an additive; and PA1 a second toner B comprising a toner particle group (b) having a second particle size distribution different from the first particle size distribution and an additive; PA1 wherein the toner particle group (a) has a smaller average particle size than that of the toner particle group (b) and the amount of additive in the first toner A is greater than the amount of additive in the second toner B. PA1 adding an additive to a toner particle group (a) having a first particle size distribution to form a first toner A; PA1 adding an additive to a toner particle group (b) having a second particle size distribution different from the first particle size distribution to form a second toner B; and PA1 mixing the first toner A with the second toner B; PA1 wherein the toner particle group (a) has a smaller average particle size than that of the toner particle group (b) and the amount of additive added to the first toner A is greater than the amount of additive added to the second toner B.
Various development methods using such toners have been proposed and put into practical use. Examples of such development methods include: the magnetic brush method described in U.S. Pat. No. 2,874,063; the cascade development described in U.S. Pat. No. 2,618,552; the powder cloud development described in U.S. Pat. No. 2,221,776; the method using conductive magnetic toner described in U.S. Pat. No. 3,909,258; and the method using various insulating magnetic toners disclosed in Japanese Patent Publication No. 41-9475.
The toners used in these development methods are generally manufactured by a pulverizing method in which a coloring agent like a dye or pigment is mixed with, and uniformly dispersed in, a thermoplastic resin serving as the binder. The mixed substance thus obtained is then finely pulverized and classified so as to provide a desired particle size.
The above manufacturing process, which can produce very excellent toners, has a problem in that there is a certain limitation regarding the range of choices for the toner materials.
For example, the above-mentioned mixed substance, which comprises the binder resin and the coloring agent, and the like uniformly dispersed therein, must be brittle enough to be finely pulverized by a manufacturing apparatus which allows economical use.
As a result, when it is finely pulverized at high speed in an actual manufacturing process, a particle group having a wide range of particle size can be formed, resulting in a so-called broadening of particle size distribution. In particular, the resulting pulverized product contains a relatively large proportion of particle groups which have undergone excessive fine pulverizing. Moreover, when actually used as a developer in copying machines, and the like, such a highly brittle toner is liable to undergo further fine pulverization or powdering.
Japanese Patent Publication No. 36-10231 discloses a toner manufacturing process based on suspension polymerization as a means for overcoming the above problem in the pulverizing method, i.e., the broadening of toner particle size distribution.
According to this process, a polymerizing monomer and a coloring agent, together with a polymerization initiator, cross linking agent, charge control agent, and other additives, as needed, are uniformly dissolved or dispersed to provide a monomer composition. The monomer composition is dispersed in a continuous phase (e.g., water phase) containing a dispersion stabilizer by using an appropriate agitator, and, at the same time, polymerization is effected, whereby toner particles having a desired particle size can be obtained. However, a toner for copying machines which was actually manufactured by the above process still needs improvement in terms of sharpness in particle size distribution, although the broadening of particle size distribution had been mitigated as compared with that in the pulverizing method.
Other toner manufacturing processes based on polymerization, for example, emulsion polymerization, precipitation polymerization, dispersion polymerization, soap-free emulsion polymerization and seed polymerization, also provide improvements in terms of broadening of toner particle size distribution. However, in these processes, the toner particles produced are fine spherical particles, so that, when toner images are transferred to the recording material, it is usually difficult to remove the remaining toner on the photosensitive member. Thus, these methods need further improvement.
One of the problems attributable to broadening of toner particle size distribution is that the way the toner flies from the developer carrier to the photosensitive drum differs depending on the particle size of the toner particles.
This will be explained with reference to FIG. 2, which shows the particle size distribution of the toner particles of an ordinary one-component-type magnetic developer prepared by the above-described pulverizing method. Toner tribo electric charge measurement was performed at three points in FIG. 2: point Y indicating the number average particle size (approximately 7 .mu.m); point X indicating a particle size smaller than the average (approximately 3.5 .mu.m); and point Z indicating a particle size larger than the average (approximately 10 .mu.m). The measurement results are shown in Table 1.
TABLE 1 ______________________________________ Toner Triboelectric Results Obtained At Three Different Toner Particle Sizes ______________________________________ Toner particle size (.mu.m) 3.5 7 10 Toner tribo electric charge (.mu.c/g) 40 10 6 ______________________________________
The results shown in Table 1 indicate the conventionally known fact that toner triboelectric charge is substantially in proportion to toner surface area (which is the reciprocal of the square of toner particle size). The way toner transfers (i.e., its developing capacity) greatly depends on the toner configuration. For example, as shown in FIG. 3, the particle size of transferable toner varies with the development contrast (i.e., the difference in potential between the latent image on the developer carrier holding toner and that on the photosensitive member). It can be seen from FIG. 3 that, the lower the development contrast, the smaller the particle size of the transferring toner.
FIG. 3 shows experimental results obtained by using the copying machine NP 6650 (manufactured by Canon K. K.). In the experiment, sampling was performed on the toner transferring onto the photosensitive member on which latent images were formed with different development contrasts (0 V, 200 V, and 400 V), and the particle size distribution of the toner was measured by a Coulter Counter.
In the above experiment, a so-called non-contact development method or jumping development method, described, for example, in Japanese Patent Publication No. 59-32375, was employed, in which, as shown in FIG. 4, an alternating electric field with superimposed DC voltage is applied between a photosensitive drum 1 and a development sleeve 3.
More specifically, by using this electric field, toner was caused to transfer from the development sleeve 3 to the photosensitive drum 1 over the gap therebetween, which was 250 .mu.m or less. A latent image with dark portions of +600 V and bright portions of 0 V was formed on the photosensitive drum 1, and a development bias voltage comprising a rectangular-wave alternating voltage of a peak-to-peak voltage of 1400 V and a frequency of 1800 Hz, and a DC voltage of +150 V, superimposed thereon, was applied to the development sleeve 3.
While in the above method an alternating electric field is superimposed as the development bias, various phenomena attributable to differences in toner particle sizes, as in the above case, are also generated in an apparatus of the type in which the development bias applied consists of a DC voltage only.
Various development devices have been proposed or put into practical uses which do not depend upon differences in toner particle size, and in which it is possible, for example, to impart triboelectric charge in a uniform manner. At present, however, no apparatus is available in which the above problem has been overcome.
Further, it is possible to effect toner triboelectric charge control by means of an additive such as silica. However, the amount of the additive in the toner is determined by the average condition of the particle size distribution of that toner, so that it is difficult to add a proper amount of additive to the toner particles composing particle size distributions.
That is, the additive is uniformly added to the entire toner, which has a broad particle size distribution. In other words, the amount of additive is not controlled by adding a relatively large amount of additive to a toner portion having a smaller particle size (a larger specific surface area) and adding a relatively small amount of additive to a toner portion having a larger particle size (a smaller specific surface area). Thus, the broader the toner particle size distribution, the less likely the toner at the ends of the particle size distribution can have the proper amount of additive.
Therefore, the difference in toner particle size between the transferring toner particles due to the variation in development contrast causes a change in the particle size distribution of the remaining toner in the developer unit after the formation of a large number of images. That is, the proper, initially set development conditions, e.g., the proper development-bias condition, are departed therefrom, resulting in a deterioration in image quality, such as a reduction in image density or fogging in non-image portions.
Further, the following problem has to be taken into account where there is a process for transferring toner images, in which a process for removing the toner remaining on the photosensitive member is usually provided: in a photosensitive-member-surface cleaning means, such as blade cleaning, it is generally difficult to remove a toner portion having a smaller particle size. This is attributable to the fact that a toner having a small particle size can pass through the interface between the photosensitive member and the blade and that, in the case of a toner having a small particle size, the adhesive force with respect to the photosensitive member, such as reflection force, is relatively large due to the higher toner triboelectric charge.
The extra toner thus allowed to remain on the photosensitive member leads to contamination of the interior of the development device, and, further, shortens the life of the cleaning blade serving as the photosensitive member cleaning means.
The toner produced by the above-described methods, based on suspension polymerization, and the like, is in the form of fine spherical particles, which means it is even easier for them to pass through the interface between the photosensitive member and the blade.
Therefore, an appropriate amount of auxiliary cleaning agent is often added when a toner is produced so that the removal of the toner having a smaller particle size from the photosensitive member may be promoted. However, like adding an additive, an amount of auxiliary cleaning agent proper for a toner portion having a smaller particle size can be excessive for a toner portion having a larger particle size and, consequently, a smaller specific surface area. As a result, the toner portion having the larger particle size is subject to a change in charging characteristics due to surface contamination. Further, the method involves device contamination due to scattering of toner, fogging in non-image portions, and the like.